Preliminary engineering design of vacuum thermal shielded Pb-Li loop section for potential nuclear fusion application

•Vacuum cover on a 1.8 m long Pb-Li loop section with a U bend for thermal shield.•Shielded pipe diameter size reduced by ∼46% through vacuum thermal shield design.•The design offers important advantage for tritium recovery and accountability.•The design offers much superior human safety from tritium radioactivity perspective. Nuclear fusion reactor design and engineering has reached into an advanced phase where the issues pertaining to radioactive tritium handling & accountability, divertor heat load mitigation, reactor size reduction & cost minimization are some of the core problems being addressed globally. Tritium breeding blanket is an important sub system of the reactor, which demands the most efficient design from many perspectives including that of tritium handling and accountability. Especially, Liquid Metal Breeder Blanket (LMBB) design consists of a complex network of heated pipes carrying Lead-Lithium (Pb-Li) Eutectic Alloy liquid metal. There are a few major issues with such pipes, namely (1) Large Space required due to substantial increase in diametric size of such pipes covered with heating elements and silica fibre based thermal insulation, (2) Loss of the bred tritium from liquid metal through permeation across the pipe wall and associated radioactive safety issues, and (3) Tritium retention in silica fibre based insulation resulting in tritium accountability problem and health hazard during maintenance duration. One possible solution to these problems could be putting a vacuum envelope over the loop pipe. The vacuum isolation will take care of thermal insulation for convection loss and at the same time, the pumping port of the envelope will collect all the lost tritium. In order to demonstrate the feasibility of the concept from thermal shielding point of view, we have prepared a conceptual and preliminary engineering design of such a vacuum envelope on a segment of the Pb-Li loop of 1 inch diameter. The loop segment includes a straight part as well as one 180° U bend too. The design is to maintain the envelop temperature less than 50 °C while the loop pipe is maintained at 350 °C. Basic calculation of thermal conductivity of low pressure air (vacuum) is done and compared with that of conventional insulation material silica based fibre felt. The fabrication, integration and maintenance schemes are also discussed in this report.